The present invention relates to the field of electromagnetic transducers and actuators, and more particularly it relates to improvements in loudspeaker drivers of the type having dual voice coils axially located in corresponding dual annular magnetic air gaps on a common axis.
In addressing fundamental design issues of dual-voice-coil dual-magnetic-gap loudspeaker drivers as related to conventional single-voice-coil drivers, the present inventors have found that the dual-voice-coil dual-gap type offers advantages with regard to linearity, efficiency, available voice coil excursion, power compression, heat dissipation and maximum sound pressure output capability. Furthermore they have found that certain benefits of the dual-coil dual-gap approach can be further enhanced by introducing shorting rings in the region of the two magnetic gaps near the voice coils.
Japanese patent 61-137496 to Okada introduces a conductive annular plate in a speaker magnet structure to prevent burning of a voice coil and to prevent an eddy current giving adverse influences to a voice coil current.
U.S. Pat. No. 5,381,483 to Grau discloses a minimal inductance electrodynamic transducer having ferromagnetic shunting rings coated with a highly conductive material to increase the induced current carrying capacity of the transducer.
U.S. Pat. No. 3,830,986 to Yamamuro discloses a MAGNETIC CIRCUIT FOR AN ELECTRO-ACOUSTIC CONVERTER having an air gap formed of a magnetic material laminated with a conductive layer for acting as shorting rings to decrease the inductance of the voice coil.
Japanese patent WO 81/02501 discloses a MAGNETIC CIRCUIT FOR AN ELECTRO-MECHANICAL TRANSDUCER OF A DYNAMIC ELECTRICITY TYPE wherein compensating coils or conductors within the magnetic gaps are supplied with signal current to prevent disturbances in the magnetic field.
Japanese patent 198208 discloses an ELECTROMAGNETIC CONVERTER wherein a magnetic ring is located in the air gap so that it can be moved axially between a circumferential yoke and a center yoke to provide good conversion efficiency by using a hollow disk permanent magnet that is magnetized in different poles at the center and external circumference.
U.S. Pat. No. 3,783,311 to Sato et al discloses a MAGNETIC DEVICE FOR USE IN ACOUSTIC APPARATUS wherein a metallic member in a voice coil gap permits the lines of magnetic force to move substantially in one direction only, for distortion reduction.
Soviet Union patent 587645/SU197801 to Rotshtein for an electromagnetic loudspeaker magnetic circuit disclose a magnetic shunt of soft magnetic material placed over a core pole piece to increase acoustic pressure by decreasing magnetic resistance.
The foregoing patents are confined to conventional loudspeaker driver/actuator construction having only a single gap and a single voice coil.
Patents that disclose dual voice coil dual magnetic gap drivers/actuators include U.S. Pat. No. 4,612,592 to Frandsen, U.S. Pat. No. 5,231,336 to Van Namen, and French patent 1,180,456 to Kritter; however these do not disclose the use of shorting rings.
U.S. Pat. No. 4,914,707 to Kato et al for a BALANCE VEHICULAR SPEAKER SYSTEM suggests attaching a shorting ring to a coil of a dual-coil dual-gap front speaker in a vehicle to decrease the high frequency impedance as an alternative to connecting a resistor in series with a rear speaker, for purposes of making the impedance of the rear speaker higher than that of the front one.
It is a primary object of the present invention to provide improvements in a dual-voice-coil/dual-magnetic-gap type transducer that will reduce harmonic distortion in the acoustic output.
It is a further object of the present invention to implement the aforementioned improvements in a manner that will reduce even order harmonic distortion including particularly second harmonic distortion.
It is a still further object of the present invention to implement the aforementioned improvements in a manner that will reduce odd order distortion including particularly third harmonic distortion.
The above-mentioned objects and have been accomplished and the advantages have been realized by the present invention as applied as an improvement to loudspeakers and other transducers of the dual-voice-coil/dual-magnetic-gap type by the addition of one or more shorting rings of high conductivity metal strategically located in the vicinity of the two magnetic gaps close to the voice coils and secured in place in fixed relationship relative to the main structure of the loudspeaker or transducer.
The shorting rings have no effect on a steady state magnetic field but act in opposition to any change in flux density or any displacement of the flux lines such as those that occur under the loading imposed when the voice coils are driven hard with audio frequency current. The location of the shorting rings determines their effect: location close to a voice coil reduces the voice coil inductance, location entirely within the magnetic flux loop centerline favors reduction of second harmonic and higher order even harmonic distortion, a centered location on the flux loop centerline, i.e. centered in the magnetic gap, favors reduction of third harmonic and higher odd order harmonic distortion, while location outside the flux loop centerline but near the voice coil acts generally to reduce harmonic distortion. Thus a plurality of rings can be differently located so as to optimally suppress both even and odd order harmonic distortion and reduce the voice coil inductance.